A combined cycle plant includes a gas turbine, an exhaust heat recovery boiler that generates steam using the heat of exhaust gas from the gas turbine, a steam turbine that is driven by the steam from the exhaust heat recovery boiler, and a condenser that turns steam exhausted from the steam turbine back into water. Water inside the condenser is supplied to the exhaust heat recovery boiler via a feed water line.
In a combined cycle plant disclosed in Patent Document 1 described below, fuel of a gas turbine is preheated using water heated in an exhaust heat recovery boiler. The exhaust heat recovery boiler includes a fuel economizer that heats water by causing water supplied to the exhaust heat recovery boiler to exchange heat with exhaust gas from the gas turbine, and an evaporator that turns water into steam by causing the water, which has been heated by the fuel economizer, to exchange heat with the exhaust gas. A heated feed water line, which supplies the water that has been heated in the fuel economizer to the evaporator, is branched partway. A diverging line that branches out from the heated feed water line is connected to a fuel preheater that heats fuel of the gas turbine. The fuel preheater heats the fuel by causing the fuel to exchange heat with the water from the diverging line. A collection line, which returns the water that has exchanged heat with the fuel, is connected to the fuel preheater. This collection line is connected to a feed water line.
In this combined cycle plant, when the flow rate of the fuel suddenly decreases significantly, an amount of heat exchanged between the fuel and the water in the fuel preheater suddenly decreases significantly. As a result, the water is returned to the feed water line while having been barely cooled by the fuel. A pressure in the feed water line is lower than a pressure in the diverging line that supplies the water to the fuel preheater. Thus, the pressure of the water that has barely been cooled by the fuel is reduced as the water enters into the feed water line or in the course of entering into the feed water line, which causes the water to flash. When the water flashes inside the feed water line, there is a risk that the line itself or a device or the like connected to the line may be damaged.
Thus, in this combined cycle plant, when a temperature of the water that has passed through the fuel preheater is higher than a predetermined temperature, the water is discharged into a condenser.